1. Field of the Invention
The present invention relates to an ultrasound apparatus which permits dynamic variation of the position of the reception focus, and in particular to such an apparatus wherein an ultrasound transducer element has an associated reception channel including a time-delay circuit for the received ultrasound echo signal, the time-delay circuit being connected to a control means for switching among various selected values of time-delay.
2. Description of the Prior Art
It is standard in ultrasound examination technology to acheive a variation of the position of the reception focus of an ultrasound array (ultrasound antenna) by so-called electronic focussing. To that end, the received echo signals for every transducer element or for a group of transducer elements of the ultrasound array are differently time-delayed. A curved reception line is thereby simulated, exhibiting different foci in accord with the degree of curvature. In phased-array apparatus, not only focussing but also a modification of the emission direction are done. This so-called swing is likewise effected via an electronic time-delay.
In dynamic focussing, an ultrasound signal is emitted and the reflected ultrasound echoes are subsequently received given successively set focus positions. In a phased-array apparatus or in multi-channel, large antennas, there is a desire to be able to switch the position of the reception focus rather frequently during the reception of an ultrasound line, i.e. dynamically. Given a large antenna having aperture correction, for example, ten focus positions per ultrasound line should be achieved. In phased-array apparatus, work is usually carried out with the full antenna. Even more than ten focus positions per ultrasound line can be desirable in this context.
When the focus positions are quickly switched during the reception of the ultrasound echo signals by a switch-over means, for example with conventional analog switches, spikes or switching clicks from these switches occur. Particularly when a depthdependent amplification means follows the switch-over means in the reception channel, can occur image disturbances.
A further disadvantage arises when the switching between the focus positions ensues too abruptly. The boundary surfaces between the focus positions can then be seen on the picture screen due to differing presentation of the fine echo structure produced by cut-off errors.
In order to avoid an abrupt switching, a method for dissolving the focused positions "softly", i.e., for example, within 10 .mu.s, is used in ultrasound apparatuses having a linear array. This procedure is meaningful given few focus positions, for example three focus positions, during the reception of the ultrasound line. The dissolve region is then short in comparison to the effective regions of the individual focus positions. Therefore, few higher-frequency components which could falsify the useful signal arise during switching. Moreover, the switch-over outlay can be economically justified given few processing channels. The number of dissolve elements required for the "soft" switching is determined by the number of reception channels multiplied by the number of focus positions. The "effective region" of a focus position is the time during which the array exhibits a constant, electronic curvature.
This procedure, however, is no longer meaningful given phased array apparatuses or given large reception antennas having many focus positions. The effective region of the individual focus position then lies in the proximity of the switch-over time so that, in the extreme case, only switching is carried out and the ultrasound echo signals are no longer received within a constant effective region.